In the drilling of oil and gas wells, concentric casing strings are installed and cemented in the borehole as drilling progresses to increasing depths. Each new casing string is supported within the previously installed casing string, thereby limiting the annular area available for the cementing operation. Further, as successively smaller diameter casing strings are suspended, the flow area for the production of oil and gas is reduced. Therefore, to increase the annular space for the cementing operation, and to increase the production flow area, it is often desirable to enlarge the borehole below the terminal end of the previously cased borehole. By enlarging the borehole, a larger annular area is provided for subsequently installing and cementing a larger casing string than would have been possible otherwise. Accordingly, by enlarging the borehole below the previously cased borehole, the bottom of the formation can be reached with comparatively larger diameter casing, thereby providing more flow area for the production of oil and gas.
Various methods have been devised for passing a drilling assembly through an existing cased borehole and enlarging the borehole below the casing. One such method is the use of an underreamer, which has basically two operative states—a closed or collapsed state, where the diameter of the tool is sufficiently small to allow the tool to pass through the existing cased borehole, and an open or partly expanded state, where one or more arms with cutters on the ends thereof extend from the body of the tool. In this latter position, the underreamer enlarges the borehole diameter as the tool is rotated and lowered in the borehole.
A “drilling type” underreamer is typically used in conjunction with a conventional pilot drill bit positioned below or downstream of the underreamer. The pilot bit can drill the borehole at the same time as the underreamer enlarges the borehole formed by the bit. Underreamers of this type usually have hinged arms with roller cone cutters attached thereto. Most of the prior art underreamers utilize swing out cutter arms that are pivoted at an end opposite the cutting end of the cutting arms, and the cutter arms are actuated by mechanical or hydraulic forces acting on the arms to extend or retract them. Typical examples of these types of underreamers are found in U.S. Pat. Nos. 3,224,507; 3,425,500 and 4,055,226. In some designs, these pivoted arms tend to break during the drilling operation and must be removed or “fished” out of the borehole before the drilling operation can continue. The traditional underreamer tool typically has rotary cutter pocket recesses formed in the body for storing the retracted arms and roller cone cutters when the tool is in a closed state. The pocket recesses form large cavities in the underreamer body, which requires the removal of the structural metal forming the body, thereby compromising the strength and the hydraulic capacity of the underreamer. Accordingly, these prior art underreamers may not be capable of underreaming harder rock formations, or may have unacceptably slow rates of penetration, and they are not optimized for the high fluid flow rates required. The pocket recesses also tend to fill with debris from the drilling operation, which hinders collapsing of the arms. If the arms do not fully collapse, the drill string may easily hang up in the borehole when an attempt is made to remove the string from the borehole.
Conventional underreamers have several disadvantages, including cutting structures that are typically formed of sections of drill bits rather than being specifically designed for the underreaming function. Therefore, the cutting structures of most underreamers do not reliably underream the borehole to the desired diameter. A further disadvantage is that adjusting the expanded diameter of a conventional underreamer requires replacement of the cutting arms with larger or smaller arms, or replacement of other components of the underreamer tool. It may even be necessary to replace the underreamer altogether with one that provides a different expanded diameter. Another disadvantage is that many underreamers are designed to automatically expand when drilling fluid is pumped through the drill string, and no indication is provided at the surface that the underreamer is in the fully-expanded position. In some applications, it may be desirable for the operator to control when the underreamer expands.
Accordingly, it would be advantageous to provide an underreamer that is stronger than prior art underreamers, with a hydraulic capacity that is optimized for the high flowrate drilling environment. It would further be advantageous for such an underreamer to include several design features, namely cutting structures designed for the underreaming function, mechanisms for adjustment of the expanded diameter without requiring component changes, and the ability to provide indication at the surface when the underreamer is in the fully-expanded position. Moreover, in the presence of hydraulic pressure in the drill string, it would be advantageous to provide an underreamer that is selectively expandable.
Another method for enlarging a borehole below a previously cased borehole section includes using a winged reamer behind a conventional drill bit. In such an assembly, a conventional pilot drill bit is disposed at the lowermost end of the drilling assembly with a winged reamer disposed at some distance behind the drill bit. The winged reamer generally comprises a tubular body with one or more longitudinally extending “wings” or blades projecting radially outwardly from the tubular body. Once the winged reamer has passed through any cased portions of the wellbore, the pilot bit rotates about the centerline of the drilling axis to drill a lower borehole on center in the desired trajectory of the well path, while the eccentric winged reamer follows the pilot bit and engages the formation to enlarge the pilot borehole to the desired diameter.
Yet another method for enlarging a borehole below a previously cased borehole section includes using a bi-center bit, which is a one-piece drilling structure that provides a combination underreamer and pilot bit. The pilot bit is disposed on the lowermost end of the drilling assembly, and the eccentric underreamer bit is disposed slightly above the pilot bit. Once the bi-center bit has passed through any cased portions of the wellbore, the pilot bit rotates about the centerline of the drilling axis and drills a pilot borehole on center in the desired trajectory of the well path, while the eccentric underreamer bit follows the pilot bit and engages the formation to enlarge the pilot borehole to the desired diameter. The diameter of the pilot bit is made as large as possible for stability while still being capable of passing through the cased borehole. Examples of bi-center bits may be found in U.S. Pat. Nos. 6,039,131 and 6,269,893.
As described above, winged reamers and bi-center bits each include underreamer portions that are eccentric. A number of disadvantages are associated with this design. First, before drilling can continue, cement and float equipment at the bottom of the lowermost casing string must be drilled out. However, the pass-through diameter of the drilling assembly at the eccentric underreamer portion barely fits within the lowermost casing string. Therefore, off-center drilling is required to drill out the cement and float equipment to ensure that the eccentric underreamer portions do not damage the casing. Accordingly, it is desirable to provide an underreamer that collapses while the drilling assembly is in the casing and that expands to underream the previously drilled borehole to the desired diameter below the casing.
Further, due to directional tendency problems, these eccentric underreamer portions have difficulty reliably underreaming the borehole to the desired diameter. With respect to a bi-center bit, the eccentric underreamer bit tends to cause the pilot bit to wobble and undesirably deviate off center, thereby pushing the pilot bit away from the preferred trajectory of drilling the well path. A similar problem is experienced with respect to winged reamers, which only underream the borehole to the desired diameter if the pilot bit remains centralized in the borehole during drilling. Accordingly, it is desirable to provide an underreamer that remains concentrically disposed in the borehole while underreaming the previously drilled borehole to the desired diameter.
In drilling operations, it is conventional to employ a tool known as a “stabilizer.” In standard boreholes, traditional stabilizers are located in the drilling assembly behind the drill bit for controlling the trajectory of the drill bit as drilling progresses. Traditional stabilizers control drilling in a desired direction, whether the direction is along a straight borehole or a deviated borehole.
In a conventional rotary drilling assembly, a drill bit may be mounted onto a lower stabilizer, which is disposed approximately 5 feet above the bit. Typically the lower stabilizer is a fixed blade stabilizer that includes a plurality of concentric blades extending radially outwardly and spaced azimuthally around the circumference of the stabilizer housing. The outer edges of the blades are adapted to contact the wall of the existing cased borehole, thereby defining the maximum stabilizer diameter that will pass through the casing. A plurality of drill collars extends between the lower stabilizer and other stabilizers in the drilling assembly. An upper stabilizer is typically positioned in the drill string approximately 30-60 feet above the lower stabilizer. There could also be additional stabilizers above the upper stabilizer. The upper stabilizer may be either a fixed blade stabilizer or, more recently, an adjustable blade stabilizer that allows the blades to be collapsed into the housing as the drilling assembly passes through the casing and then expanded in the borehole below. One type of adjustable concentric stabilizer is manufactured by Andergauge U.S.A., Inc., Spring, Tex. and is described in U.S. Pat. No. 4,848,490. Another type of adjustable concentric stabilizer is manufactured by Halliburton, Houston, Texas and is described in U.S. Pat. Nos. 5,318,137; 5,318,138; and 5,332,048.
In operation, if only the lower stabilizer was provided, a “fulcrum” type assembly would be present because the lower stabilizer acts as a fulcrum or pivot point for the bit. Namely, as drilling progresses in a deviated borehole, for example, the weight of the drill collars behind the lower stabilizer forces the stabilizer to push against the lower side of the borehole, thereby creating a fulcrum or pivot point for the drill bit. Accordingly, the drill bit tends to be lifted upwardly at an angle, i.e. build angle. Therefore, a second stabilizer is provided to offset the fulcrum effect. Namely, as the drill bit builds angle due to the fulcrum effect created by the lower stabilizer, the upper stabilizer engages the lower side of the borehole, thereby causing the longitudinal axis of the bit to pivot downwardly so as to drop angle. A radial change of the blades of the upper stabilizer can control the pivoting of the bit on the lower stabilizer, thereby providing a two-dimensional, gravity based steerable system to control the build or drop angle of the drilled borehole as desired.
When an underreamer or a winged reamer tool is operating behind a conventional bit to underream the borehole, that tool provides the same fulcrum effect to the bit as the lower stabilizer in a standard borehole. Similarly, when underreaming a borehole with a bi-center bit, the eccentric underreamer bit provides the same fulcrum effect as the lower stabilizer in a standard borehole. Accordingly, in a drilling assembly employing an underreamer, winged reamer, or a bi-center bit, a lower stabilizer is not typically provided. However, to offset the fulcrum effect imparted by to the drill bit, it would be advantageous to provide an upper stabilizer capable of controlling the inclination of the drilling assembly in the underreamed section of borehole.
In particular, it would be advantageous to provide an upper stabilizer that engages the wall of the underreamed borehole to keep the centerline of the pilot bit centered within the borehole. When utilized with an eccentric underreamer that tends to force the pilot bit off center, the stabilizer blades would preferably engage the opposite side of the expanded borehole to counter that force and keep the pilot bit on center.